The present invention relates to a data decoding and reproducing method for high density information storage, and a data decoding and reproducing circuit and an information storing and reproducing apparatus for implementing such a method.
In implementing high density information storage, it is demanded to demodulate recorded information data from a signal reproduced from a recording medium, with high reliability. In high density magnetic recording and reproducing apparatuses represented by magnetic disk drive apparatuses, data demodulation with high reliability becomes difficult. Because an increase of intersymbol interference and a remarkable degradation in signal quality, i.e., signal-to-noise ratio is caused in the reproduced signal as the recording density is increased.
In order to overcome such a technical problem, the partial-response maximum-likelihood detection (PRML) technique obtained by combining the partial-response equalization technique with the maximum likelihood detection technique is widely used. In particular, in magnetic recording, a combination of the partial response class 4 (PR4) or its extended equalization technique with maximum likelihood data detection implemented by the Viterbi algorithm or the like has been applied as a signal processing technique that is more suitable for high density data recording and reproduction. On the other hand, in optical recording and reproducing apparatuses or the like, a combination of the partial response class 1 or its extended equalization technique with maximum likelihood data detection has been proposed as a suitable data signal processing technique.
Such a PRML technique is widely known as a data detection/demodulation technique that provides means for effectively utilizing signal power of the reproduced signal sequence and decoding the most-likely data sequence from the noisy reproduced signal sequence. The PRML technique is implemented as extremely small-sized, highly integrated circuits with the advance of the semiconductor technologies. Such integrated circuits are mounted on a lot of information recording and reproducing apparatuses, thus bringing about effects that are effective in achieving high density recording. The principle of such a PRML data demodulation technique is to improve the reliability of data modulation against noise on a reproduced signal by effectively utilizing intersymbol interference (ISI) the reproduced signal waveform has.
In the conventional PRML data detection/demodulation technique, it is supposed that noise component contained in a reproduced signal at the detection point can be regarded as stationary white noise. In other words, it is supposed that the noise variation components contained in the reproduced signal behave as stochastic random variations that are completely independent of each other. In addition, suppose that the noise component at any point of the reproduced signal follows the same statistical property, it is ensured that the ML (maximum-likelihood) decoder can decode the most-likely data sequence. In the signals reproduced from an actual high density information storage apparatus, however, it is extremely difficult to maintain these suppositions for the noise variation component. In particular, in reproduced signals in high density magnetic storage apparatuses, existence of a medium noise caused by disorder of magnetization bit cell written onto the medium becomes remarkable. In this case, the noise components exist locally depending on the recorded magnetization state on the recording medium. Therefore, distribution of the media noise on the reproduced signal changes greatly depending on the recorded magnetization pattern (data sequence pattern), and it can be regarded as so-called nonstationary noise variations with strong data dependency and correlation. As the premise for the noise components greatly collapses, the reliability of data detection in the PRML data demodulation technique gradually degrades.